Corrosion resistant steel and equipment



Patented Aug. 31,1948

UNITED STATES PATENT: OFFICE CORROSION RESISTANT STEEL EQUIPMENT AND Louis Secondo Benzoni, Port Colborne, Ontario,

Canada, asslgnor to International Nickel Company, Inc., New York, N. Y., a corporation of Delaware No Drawing.

Application March 20, 1946, Serial No. 655,893. In' Canada February 12, 1946 U The present invention relates to alloy steels, and chemical equipment made therefrom, which lutions, e. g., in the electrolytic refining of nickel with a sulfate-chloride electrolyte such as described in my co-pending United States application, Serial No. 472,471, now U. S. Patent No. 2,394,874. However, the chloride-bearing solutions have to be conveyed into and out of containers, tanks, vats, etc., by means of pumps and through valves, pipes, conduits and the like, for which uses non-metallic materials are not suitable. A need existed for an alloy steel highly resistant to corrosive acidic chloride-bearing solutions, particularly impure chloride solutions which are very corrosive, which steel could be used for the manufacture of pump castings, valve castings,- pipes, containers and like chemical equipment or apparatus, Although many attempts were made to provide alloy steels highly resistant to corrosive aqueous acidic chloride- For example, mastic-' 11 Claims. (01. 75-128) acidic solutions, and particularly suitable for vmaking chemical equipment such as pump castings, valve castings, cast pipes, containers and the like.

It is a further objectof the present invention to provide chemical equipment for handling corchloride-bearing solutions, including sulfate-chlohearing solutions, none, as far as I am aware, was

successful when carried into commercial use.

I have discovered an alloy steel highly resistant to corrosive aqueous acidic chloride-bearing solutions, includin the very corrosive impure sulfatechloride and all-chloride solutions, having a critical composition within a narrow range and containing nickel, chromium and molybdenum.

It is an object of the present invention to provide an alloy steel, and articles made therefrom,

highly resistant to aqueous acidic chloride-bearing solutions.

It is another object of the invention to provide an alloy steel highly resistant to extremely corrosive impure sulfate-chloride and all-chloride,

f position be maintained within the range-of com rosive aqueous acidic chloride-bearing solutions,

which equipment is made of an improved alloy steel highly resistant to such solutions.

Other objects of the invention will become apparent to those skilled in the art from the following description.

The alloy steel provided by the present invention is highly resistant to very corrosive aqueous ride and all-chloride solutions. and comprises 21% to 23% nickel; 21% to 22% chromium, 5% to 6% molybdenum, not more than about 0.15% carbon, preferably not more than 0.12% carbon, and the balance substantially all iron. It is desirable that the chromium content not exceed the nickel and that the nickel content increase with increasing chromium content within the aforementioned ranges, Preferably, the nickel content is maintained within the range oi. 22% to 23%. It is essential that the foregoing controlled compositions be employed as a substantialdeparture from said ranges results in failure to achieve the desired results, includin the high corrosion resistance provided by the alloy steels having A compositions within the aforementioned ranges. The alloy steels having the foregoing composition are characterized by a completely austenitic structure. It is essential that the steels of the present invention be devoid of a second phase such as the well-known sigma phase which is often present in high-chromium iron-base compositions. The mere absence in an austenitic alloy steel of a second phase such asthe sigma phase does not in itself mean that the alloy steel will be corrosion resistant to the aforementioned acidic aqueous chloride-bearing solutions, including impure sulfate-chloride and all-chloride solutions. It is also essential in order to obtain high corrosion resistanceto such acidic aqueous chloride solutions that the completely austenitic compositions set forth hereinbefore. A feature of said composition is the critical proportioning of the nickel content, to the chromium and molyba 3 denum contents. Carbon is not an essential element and is tolerated mainly because it is commercially impracticable to produce the metal composition without some carbon being present. It is preferred to maintain the carbon content as low as commercially feasible. Because some carbon is usually present, the compositions have been referred to as a steel. The present invention does not exclude the presence of the usual small amounts, say up to a total of about 1% or 1.5%, of other elements which are commonly present in or added to steels, especially of the austenitic ty as a result of good steel-makin practice or for known purposes, Illustrative examples inelude-the normal content of sulfur, phosphorus, manganese, silicon, copper, titanium, columbium, etc. As silicon and manganese strongly promote the formation of the sigma phase, it is preferable that the amounts of these elements be held to a minimum or even avoided entirely whenever practicable. Titanium or columbium are frequently employed to fix carbides and may be present, especially if the carbon content is suinclently high to promote excessive carbide preride solutions such as the impure anolyte or elec-' troiyte used in the process described in my copending application, Serial No. 472,471,,now U. 8. Patent No. 2,394,874, has the following composition:

Per cent Nickel 22.8 Chromium 21.1 Molybdenum 5.4

Carbon 0.11.

' Iron Balance Pumps. and conduits made of the foregoing composition have given satisfactory results when used in the process of my above-mentioned application to convey the impureelectrolyte from the holding tank to the unit for the removal of impurities such as iron, arsenic and lead. Compositions within the ranges s et forth hereinbefore have also given, good results in the carrying out of the process described in the aforementioned patent application when used as pipes, pumps. valves, etc., for conducting and treating the electrolyte from the electrorefining tanks to tank No. 15 in Fig. 7 of my co-pending patent application, Serial No. 472,471, now U, 8. Patent No. 2,394,874.

In order to assure the corrosion resistance contemplated by the invention, it is highly desirable that the alloys be in the heat-treated condition resulting from an annealing treatment followed by a rapidcooling, e. g.. a water quench. Sucha heat treatment is particularly desirable when carbon is present in amounts sufficient to result in the precipitation of carbides at the grain boundaries. The heat treatment also assures the absence of a second phase such as the sigma phase. A suitable annealing treatment comprises holding the alloy or the casting at a temperature of about 2150 to 2250 F. for periods of about 15 minutes to one hour depending on 4 the cross-sectional size followed immediately by a water quench.

In order that those skilled in the art may have a better understanding of the invention and a better appreciation of the critical nature of the composition and of the improved corrosion resistance obtained by the present invention, the corrosion resistant properties of compositions contemplated by the invention are compared with other compositions in the tables set forth hereinafter. Tables I and II give the percentage of carbon and various alloying elements present, in addition to iron, in the compositions. All compositions, i. e., No. 1 to No. 17, were given the same heat treatment. described hereinbefore, i. e., annealed at about 2250" F. followed immediately .by a water quench. I

Table I Composition Steel No. Per Per Per Structure cont cent cont cent i Cr Mo C 22.0 21.0 0.0 0.12 autumn. 21.0 22. 5. 5 0. 12 0. 21. 0 23. 0 5. 5 0.12 Austenite-l-sigma phase. 21. 0 22.0 6. 5 0.10 Do. 22.0 21.0 6.9 0.00 D0. 20. 2 2i. 7 3.0 0. 12 All austenite. 25.0 20. 0 5. l 0. 11 D0. 30.0 24.0 5.5 0.10 Austenite-l-aigma phase. 35. 0 25.0 5. 0 0. 20 Do. 12.0 20. 0 4. 0 0. D0. 12.0 20. 0 5. 0 0. 10 Do. 12.0 20.0 8.0 0.10 D0.

Table II Composition Steel No. Per For For Per Per Per cent cent cent cent cent cent Ni Cr Mo Cu Si 13 22 10 3.5 0.07 1.0 1.25 All austenite. 14 22 10 3. 6 0. 07 l. 0 Do. 24 22 3.0 0.07 1.75 3.25 Do. 16 20 3.0 0.07 5.0 D0. 17 15 3.0 n. d. Do.

I Not determined estimated between 0.2% and 0.3% carbon.

Steels No. 1 and No. 2 are properly proportioned DOSES.

sulfate-chloride acidic aqueous solution containing about 45 grams per liter of nickel, 0.25 gram per liter of copper, 0.1 gram per liter of iron, 30 grams per liter of sodium ion, 30 grams per liter of chloride ion, grams per liter of sulfate ion,

and 20 grams per liter of boric acid, and having a pH of about 3. These steels were moved through the solution at a rate of about 250 feet per minute for the same interval of time. The solution was maintained at a temperature of to F. The corrosion rate in milligrams per square decimeter per day (M. D. D.) was determined and the presence or absence of pitting noted on all steels. The results of the tests are summarized in the following Table 111.

It will be observed from Table III that only the compositions made in accordance with the present invention, 1. e., steels No. 1 and No. 2, had

a low corrosion rate and did not exhibit pitting. All the other compositions, even those having compositions only slightly outside the range contemplated by the invention, had very much poorer resistance to corrosion, as evidenced by the high corrosion rate and by pitting. In general, the steels contemplated by the present invention and articles made therefrom exhibit, under the aforementioned test conditions, corrosion rates which usuallydo not exceed about 15 to 20 M. D. D.

Examples of other electrolytes in which the compositions contemplated by y the -invention exhibited satisfactory corrosion resistance include impure acidic nickel-containing sulfatechloride or all-chloride aqueous solutions aerated to saturation, having a pH of 4.7, a temperature of 130 F. and containing 0.25 gram per liter of copper-and 0.06 gram per liter of iron; acidic nickel-containing sulfate-chloride or all-chloride aqueous solutions having under slightly reducing conditions a pH of 4.8, a temperature of 140 F. and containing 0.15 gram per liter of copper and 0.0003'gram per liter of iron; acidic nickel-containing sulfate-chloride or all-chloride aqueous solutions (under slightly reducing conditions) having a pH of 5.3, a temperature of 150 F. and containing 0.005 gram per liter of copper and 0.0005 gram per liter of iron; etc.

It will be appreciated that the consistent production of steels within the range of compositions contemplated by the invention requires strict control of the composition during the production of the steel in order that it fall within the specified ranges. This is particularly true of the chromium content. Allowance must be made As noted hereinbefore, the present invention is particularly suitable for the production of cast chemical equipment subjected in use to contact with impure sulfate-chloride or all-chloride aqueous solutions, including electrolytes. solutions include the impure electrolytes em- Such render solutions impure.

ployed in the process described in my aforementioned application. The impure solutions exhibit, in general, more. severe corrosive action than pure solutions. Thus. arsenic and/or lead each in amounts of 0.0002 gram per liter or more and copper and/or iron each in amounts of 0.0005 gram per liter. or more are considered to Sulfate-chloride and all-chloride acidic solutions containing nickel and/or cobalt are much more corrosive when they also contain such impurities as iron and/or copper. -Iron is particularly corrosive when present in amounts of about 0.005 gram per liter or more, especially when the iron is present in both the oxidized (ferric) and reduced (ferrous) forms as, for instance, during the oxidationv of ferrous to ferric iron in chloride-bearing acidic aqueous solutions. Copper present in amounts of about 0.02 gram per liter or more renders chloride-bearing aqueous acidic solutions much more corrosive. The corrosive action due to the presence of copper is particularly severe when both the reduced (cuprous) and oxidized (cupric) forms are present as is the case during aeration of impure electrolytes to oxidize the iron. Aeration of acidic chloride-bearing nickel solutions has been found to accelerate considerably corrosion in such solutions, and the compositions of the present invention exhibit excellent corrosion resistance in aerated solutions, including aerated impure solutions. Excellent corrosion resistance has been obtained by the present invention in acidic chloride bearing nickel containing aqueous solutions having a'pH between about 2 and about 5.5.

' Excellent corrosion resistance is obtained by the present invention in the corrosive chloride-bearing acidic aqueous-solutions containing at least about 2 grams per liter of chloride ions. With increasing chlorldeion concentration, the corrosiveness will increase but the corrosive act-ion reaches a maximum and generally remains substantially the same regardless of further increases in chloride concentration whether in'a sulfatechloride solution or an all-chloride solution. Thus, the maximum corrosion of an acidic sulfate-chloride nickel electrolyte such as described in my aforementioned application is reached at a sodium chloride content of approximately 30 grams per liter. Addition of chlorides in excess of this amount or conversion to all-chloride solutions of equalchloride concentrations does not increase the corrosive action of the acidic solution. The 'terms. "sulfate-chloride" and all-chloride, employed herein, refer to the predominant ne atively-charged ions in the aqueous solution. Sui fate-chloride solutions are those solutions'containing substantial amounts of sulfate ions in addition to chloride ions whereas all-chloride solutions are substantially devoid of sulfate ions. Of

acidic sulfate-chloride or all-chloride aqueous sO- lutions, and especially impure acidic solutions. To demonstrate the high corrosion resistance obtained by the present invention. it has been illus- Although the present invention has been described in conjunction with preierredembodimerits, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope or the appended claims.

I claim:

1. A cast corrosion resistant metal article containlng 22% to 23% nickel, 21% to'22% chrmium, to 6% molybdenum, not exceeding 0.15% carbon and the balance. essentially iron, and having an austenitic structure devoid of a sigma phase and ofpreciplta-ted carbide at the grain boundaries, said article being in a condition resulting from an annealing treatment at about 2250 F. followed by rapid cooling.

2. A cast corrosion resistant metal articlecon taining 21% to 23% nickel, 21% to 22% chromium, 5% to 6% molybdenum, not exceeding 0.15% carbon and the balance essentially iron, and having an. austenitic structure devoid of a sigma phase and of precipitated carbides at the grain boundaries, said article being in a condition resulting from an annealing treatment at 4 about 2250 F. followed by quenching.

as the chromium content, 5% to 6% molybdenum,

not exceeding 0.15% essentially iron, and having an austenitic structure devoid of a sigma phase.

5. A corrosion resistant metal article comprised of an alloy containing 21% to 23% nickel, 21% to 22% chromium, 5% to 6% molybdenum, not exceeding 0.15% carbon and the balance essentially iron, said alloy having an austenitic structure substantially devoid of a sigma phase and of precipitated carbides at the grain boundaries and being in a condition resulting from an annealing treatment at approximately 2250 F. followed, byrapid cooling.

6. Chemical equipment subjected in use to contact with a corrosive acidic chloride-bearing aqueous solution comprised of a cast alloy containing 21% to 23% nickel, 21% to 22% chromium, 5% to 6% molybdenum, not exceeding 0.15% carbon and the balance essentially iron, said alloy having an au'stenitic structure substancarbon and the balancetially devoid of a sigma phase and of precipitated carbides at the grain boundaries and being in a condition resulting from an annealing treatment at about 2250 F. followed by rapid cooling.

7. Chemical equipment subjected in use to contact with a corrosive acidic chloride-bearing aqueous solution comprised oi an annealed and rapidly-cooled alloy containing 22% to 23% nickel, 21% to 22% chromium, 5% to 6% molybdenum, not exceeding 0.15% carbon and the balance essentially iron, and having an austenitic structure devoid cl 9. sigma phase.

8. Chemical equipment subjected in use to contact with a corrosive acidic chloride-bearing aqueous solution comprised oi an alloy having an austenitic structure devoid of a sigma phase and precipitated carbides at the grain boundaries and containing chromium within the range of 21% to 22% chromium, nickel in an amount at least equal to the chromium content and within the range of 21% to 23%, molybdenum within the range of 5% to 6%, not exceeding 0.15% carbon and the balance essentially iron, said alloy being in a condition resulting from an annealing treatment at about 2250 F. followed by quenching.

9. An alloy having a single-phase austenitic structure and comprised of 22% to 23% nickel, 21% to 22% chromium, 5% to 6% molybdenum, not exceeding 0.15% carbon and the balance essentlally iron.

10. An alloy having an austenitic structure devoid of a sigma phase and comprised of 21% to 23% nickel, 21% to 22% chromium, 5% to 6% molybdenum, not exceeding 0.15% carbon and the balance essentially iron.

11. An alloy having an austeniticstructure devoid of a sigma phase and comprised of chromium within the range or 21% to 22%, nickel in an amount at least equal to the chromium content and within the range of 21% to 23%, molybdenum Within the range, 01' 5% to 6%, not exceeding 0.15% carbon and the balance essentially iron.

. LOUIS SECONDO RENZONI.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Country Date Great Britain July 5, 1939 OTHER REFERENCES Number 

